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DB2 - Problem description

Problem IC83616 Status: Closed

OPTIMIZER UNDERESTIMATES THE NUMBER OF EXPECTED ROWS IN A SORT /GROUP BY
OPERATOR, WHICH LEADS TO A BAD PERFORMING ACCESS PLAN

product:
DB2 FOR LUW / DB2FORLUW / 970 - DB2
Problem description:
A sample statement having a WHERE clause as shown below will 
generate a plan where the expected row count in SORT / GROUP BY 
operator is drastically less compared to actual number of rows. 
 
statement: 
 
SELECT... 
WHERE tab1.col1 = 'xyz' 
   GROUP BY tab2.col2 , 
tab1.col1 ,.... 
 
Note: col1 is used in the GROUP BY clause 
 
Plan generated will show estimated row count in SORT/GROUP BY 
as 3 where the actual count is 13923. 
 
         0,000176497 
              1 
           NLJOIN 
           (  16) 
           1221,82 
             NA 
         /---+----\ 
   0,000530024   0,332997 
        1            1 
     TBSCAN       IXSCAN 
     (  17)       (  23) 
     1214,24      15,1398 
       NA           NA 
       |            | 
        3           -1 
      5469          NA 
     TEMP     INDEX: SCHEMA1 
     (  18)    IDX1 
     1214,23        Q42 
       NA 
       | 
        3 
      5469 
     GRPBY 
     (  19) 
     1214,23 
       NA 
       | 
        3 
      13923 
     TBSCAN 
     (  20) 
     1214,23 
       NA 
       | 
        3 
      13923 
     SORT 
     (  21) 
     1214,23 
       NA 
       | 
      19109 
      19109 
     TBSCAN 
     (  22) 
     1209,64 
       NA 
       | 
      19109 
       NA 
 TABLE: SCHEMA1 
   TABX 
       Q5
Problem Summary:
**************************************************************** 
* USERS AFFECTED:                                              * 
* ALL                                                          * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* See Error Description                                        * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade to DB2 Version 9.7 Fix Pack 7.                       * 
****************************************************************
Local Fix:
available fix packs:
DB2 Version 9.7 Fix Pack 7 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 8 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 9 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 9a for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 10 for Linux, UNIX, and Windows

Solution
First fixed in DB2 Version 9.7 Fix Pack 7.
Workaround
not known / see Local fix
BUG-Tracking
forerunner  : APAR is sysrouted TO one or more of the following: IC86792 
follow-up : 
Timestamps
Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :
21.05.2012
14.12.2012
14.12.2012
Problem solved at the following versions (IBM BugInfos)
9.7.FP7
Problem solved according to the fixlist(s) of the following version(s)
9.7.0.7 FixList